The present invention relates to a method for manufacturing a cylinder for-an internal combustion engine such as a small air-cooled two-stroke gasoline engine which is suited for use in a portable working machine, and in particular to a method which enables an undercut portion of the cylinder such as a scavenging port to be rationally formed at the occasion of manufacturing the cylinder by means of a die casting method such as a high-pressure die casting method.
A small air-cooled two-stroke gasoline engine to be used in a portable power working machine is, as seen for instance from Japanese Patent Unexamined Publication S58-155114, generally formed of an aluminum alloy and constituted by an integral body consisting of a main body having a cylinder bore for allowing a piston to be fitted therein, a head portion provided with a squish dome-shaped combustion chamber which is formed therein, and a plurality of cooling fins which are projectingly formed all over the outer wall of the integral body.
The cylinder bore is provided with an air-fuel mixture inlet port and also with an exhaust port, which are to be closed or opened by the movement of the piston, these air-fuel mixture inlet port and exhaust port being disposed to face to each other and to disagree in level from each other. A plurality of hollow scavenging passages, each being spaced away from these air-fuel mixture inlet port and exhaust port by an angle of 90 degrees and having an inner wall of predetermined thickness, are formed along with the cylinder bore. The downstream end portion (upper end portion) of each hollow scavenging passage is constituted by a scavenging port, thereby providing a pair of scavenging ports disposed opposite to each other, which are designed to be opened or closed by the movement of the piston and are inclined somewhat upward in the direction opposite to the exhaust port of the cylinder bore.
The cylinder disclosed in the aforementioned Japanese Patent Unexamined Publication S58-155114 is a so-called binary fluid scavenging type cylinder where a pair of scavenging ports are symmetrically formed with respect to the longitudinal section taken along the middle of the exhaust port. Additionally, a so-called quaternary fluid scavenging type cylinder where a pair of scavenging ports are additionally provided therewith is also known (see Japanese Patent Application H10-203750 which belongs to the same assignee as that of the present application).
As for the type of the scavenging passage, there are known a hollow scavenging passage provided with an inner wall as shown in Japanese Patent Unexamined Publication S58-155114, a scavenging passage having no inner wall (the side facing the cylinder bore is opened) or a scavenging passage provided with a half-wall having a predetermined thickness as disclosed in Japanese Patent Application H10-203751 which belongs to the same assignee as that of the present application wherein the scavenging passage is provided at a lower portion thereof with an opening extending in the longitudinal direction of the scavenging passage while leaving the half-wall at an upper portion thereof so as to allow an air-fuel mixture being introduced into the scavenging port from the crank chamber via the scavenging passage to be contacted with the skirt portion of the piston.
In the method of manufacturing a cylinder provided with an inner wall-type (or a half-wall-type) hollow scavenging passage in particular among the aforementioned cylinders for a two-stroke internal combustion engine by means, of a die casting method such as a high-pressure die casting method which enables cast moldings of high dimensional precision to be produced at low cost, the scavenging port portion of the scavenging passage which constitutes an undercut portion in the casting of the cylinder is generally formed by a method wherein a raw cylinder body is cast-molded with the scavenging port portion (constituting an undercut portion) thereof being left closed, and thereafter, the closed scavenging port portion is cut out by mechanical means (see Japanese Patent Unexamined Publication S58-155114), since a collapsible core cannot be employed under a high pressure.
In the case of the aforementioned method to cut out a scavenging port by mechanical means after the casting of raw cylinder body however, since the space allowing a cutting tool to be inserted into a working portion is very narrow, it is very difficult to perform the mechanical working and to enhance the working precision of the scavenging port. Since the performance of an engine, in particular, a two-stroke internal combustion engine is greatly influenced by the size and configuration of the scavenging port as well as by the working precision thereof, the aforementioned problem is very important.
It may be conceivable to manufacture a cylinder provided with an inner wall-type hollow scavenging passage by means of a die casting method employing an insert core to be inserted into the scavenging port portion. In this case however, since the inserted part is left in the cast product, the heat conductivity of the product is deteriorated and at the same time, various problems such as the deformation or peeling of the inserted part may be caused to occur.
The present invention has been made to overcome the aforementioned problems, and therefore an object of the present invention is to provide a method for manufacturing a cylinder for an internal combustion engine, which makes it possible to manufacture the cylinder by means of a die casting method such as a high-pressure die casting method at low cost and in high precision without raising problems such as the deterioration of heat conductivity or the deformation or peeling of the inserted part.
With a view to realize the aforementioned object, the present invention provides a method of manufacturing a cylinder for an internal combustion engine, which is directed to the manufacture of a cylinder provided with an inner wall-type hollow scavenging passage having a scavenging port to be opened or closed by the movement of piston, the method being featured essentially in that the cylinder is manufactured by means of a die casting method using an insert core comprising a cylindrical body having substantially the same outer diameter as the diameter of bore of the cylinder desired to obtain, and a scavenging port-forming portion projecting radially outward from the cylindrical body and having substantially the same cross-sectional configuration as that of the scavenging port.
According to a preferable embodiment of the method according to the present invention, the cylinder is manufactured by a process wherein the insert core is externally attached to a bore-core die, the resultant casting die is then employed to cast-mold a raw cylinder body with the insert core being left remained therein, the cylinder bore of the resultant raw cylinder body is then bored to remove a cylindrical portion of the insert core, and the scavenging port-forming portion of the insert core which is left remained in the raw cylinder body is removed by making use of a press, etc.
According to a more preferable embodiment of the present invention, a parting agent is coated or plated on the outer surface of at least the scavenging port-forming portion of the insert core, thereby forming a mold-releasing layer prior to the step of die casting.
On the other hand, the insert core according to the present invention that can be employed in the aforementioned manufacturing method is featured in that it comprises not only a cylindrical body having substantially the same outer diameter as the diameter of bore of the cylinder desired to obtain, but also a scavenging port-forming portion projecting radially outward from the cylindrical body and having substantially the same cross-sectional configuration as that of the scavenging port.
According to a preferable embodiment of the present invention, the insert core is formed as an integral body by means of a die casting method using, as a raw material, the same kind of aluminum alloy as that of the cylinder desired to obtain, and a parting agent is coated or plated on the outer surface of the insert core thereby to form the mold-releasing layer.
As for the material for the insert core, it is not limited to the aforementioned aluminum alloy, but any other iron family metals can be employed. If the same kind of aluminum alloy as that of the cylinder is employed as a material for the insert core, the content of an additive such as silicon in the insert core may be increased larger than that of the cylinder, thereby preventing the generation of a fusion bonding between the insert core and the cylinder.
As for the parting agent to be coated or plated on the outer surface of the insert core, chromium, nickel, carbon, etc. can be employed. The coating or plating of these parting agents may be performed using an electrolytic plating or a vapor deposition, thus forming a mold-releasing layer.
According to a preferred embodiment of the method of manufacturing a cylinder for an internal combustion engine by making use of an insert core of the present invention, it is possible, due to the employment of the insert core, to utilize a high-pressure die casting method which enables to obtain a cast product of high dimensional precision. Additionally, since the cylindrical portion of the insert core can be removed by way of a rough boring of the cylinder bore after the die casting, and since the scavenging port-forming portion of the insert core that cannot be removed by the rough boring can be removed by making use of a press after the die casting, it is possible to make the resultant product completely free from any inserted part. As a result, a cylinder can be manufactured in higher precision and at lower cost as compared with the conventional method of cutting out the scavenging port portion by mechanical means after die casting or with the conventional manufacturing method by means of die casting where an insert core to be inserted into the scavenging port portion is employed. At the same time, the aforementioned problems of the deterioration of heat conductivity, and the deformation or peeling of the inserted part due to the remnant of the inserted part in the cast product (cylinder) can be prevented to occur.